The present invention relates to the improvements and developments in roll forming, particularly in the roll forming of fibre reinforced thermoplastic (FRTP) or so-called, "composite" sheet materials.
Throughout-this specification, reference is made to the terms "pitch" and "deformation length" in relation to roll forming techniques and to "melting temperature" and "recrystallisation temperature" in respect of thermoplastics. It is understood that the terms are defined as follows:
"Pitch" is the centre distance between two consecutive roll stands where a series of roll forming stations are provided in tandem.
"Deformation length" is the distance to which portions of the element being formed are deformed out of register from their original position prior to entering the current roll forming station.
"Melting temperature" is the temperature range (which could be as large as 30 degrees Celsius) at which the plastic undergoes a phase change from a solid to liquid, upon heating.
"Recrystallisation temperature" is the temperature at which the semi-crystalline or crystalline matrix begins to form crystals upon cooling from the fully molten amorphous state. The onset of re-crystalisation from the fully molten state takes place over a temperature "window-range", which is dependant on the rate of cooling and the final achievable degree of crystallinity of the polymer matrix.
The benefits arising from the ability to form thermoplastic materials in their molten state (thermoforming) are well recognised. They include the ability to form sheet materials (including FRTP sheet) to relatively fine tolerances and quality standards as a raw material and then use these products to create geometric forms which are specifically applicable to the intended purpose.
It is expected that efficient structures, particularly those formed from composite or FRTP materials, can be created which will have optimal strength to weight ratios and other desirable qualities and that these structures are able to be produced more adequately in a more convenient and controllable manner.
Notwithstanding the benefits which arise from the use of FRTP or composite materials in producing geometric forms significant difficulties have been encountered in achieving the objective of forming such materials in such a way which does not lead to a degradation in the physical structure (and the associated undue weakening of the material) and thus the formed element.
In the past, attempts have been made to utilize the roll forming technique in the forming FRTP composite sheets. These attempts have centred on two main themes both of which have proved unsatisfactory.
One method involves enclosing the whole roll forming equipment in an oven where the temperature is controlled so that it remains above the melting temperature of the FRTP composite strip. In this manner, the molten strip is then pulled through the consecutive roll forming stations, as opposed to the accepted practice of using the rotation of the roll forming dies to progressively drive the material through the forming process. The necessity of the molten strip to be pulled through the roll forming equipment places severe limitations on the speed at which the material can be formed.
Difficulties are experienced in passing the floppy FRTP material through the roll forming equipment and in holding the formed element in its desired shape while the material cools to sufficient temperature such that it remains stable.
The second method involves localised heating of the FRTP material strips in the regions where the deformation is expected to occur during the roll forming process. Once the relevant regions have been heated to above the molten temperature of the FRTP material, the strip is then passed through the roll forming equipment. Heating elements are positioned along the path the FRTP strip takes while being formed to maintain the FRTP material above its melting temperature.
This method results in uncontrolled delamination of the FRTP material as the deformation occurs as well as fracture of the reinforcing fibers in the deformation area due to the requirement that the FRTP material needs to flow generally rather than locally during a forming process. This can only be achieved by maintaining the whole FRTP strip in a molten state during the forming process.
Both of the described methods result in undesirable migration of the fibers within the deformation region, the former method due to the excessive tensions required to pull the FRTP strip through the roll forming equipment in conjunction with the high forming temperatures used and the later due to the high localized strains induced in the reinforcing fibers in the deformation region.